Color and gray value digital images are both composed of picture elements (pixels), where each pixel is represented by multiple binary bits that define either a color or a gray level. In order to represent such an image on a bi-level printer, the individual color or gray level pixels are typically converted to binary level pixels through use of a digital halftoning process.
In addition to digital halftoning, the images are compensated for toner explosion conditions. Toner explosion is a problem which occurs in laser or light emitting diode (LED) printers. If uncorrected, certain line widths of toner may have parts of the line appear to be exploded and scattered down the page as illustrated in FIG. 1 at 58. The toner explosion problem is related to the amount and placement of toner on the media and the moisture content of the media. Laser printers typically statically attach toner to media and then permanent fuse or melt the toner into the media with a fuser, which heats and presses the media. Moisture in the media attempts to escape when the media is heated and pressured in the fuser of the printer that can cause the toner to relocate (i.e., explode and scatter) down the media. Large areas of toner may have the same toner explosion problem, but with lots of other toner around, the toner typically cannot relocate down the page. Toner explosion can also occur at the bottom edge of any black area.
Template matching is typically performed on 1-bit data to essentially estimate what the original smooth shapes of the image features were in the grayscale image (i.e., prior to halftoning) and to identify toner explosion conditions. Template matching compares a window (e.g., 7×9 pixels) of the 1-bit data with hundreds of templates to identify features in the window and to identify toner explosion conditions. The window is moved around the entire 1-bit image to identify features and toner explosion conditions throughout the entire image. All bits in the image are processed. If a template matches the window, the center bit of the window is replaced. The processed image has jagged edges removed or reduced and toner explosion conditions eliminated or substantially reduced in the printed output. Typically, template matching is performed by dedicated hardware.